The present invention relates to a mechanism for scanning the inner periphery of a hollow cylindrical drum included in an image forming apparatus.
Some of laser printers, laser plotters and facsimile apparatuses available today have a scanning mechanism of the type having stationary optics and rotary optics which is disposed in a hollow cylindrical drum. A film or similar photoconductive member is fitted on the inner periphery of the drum. The rotary optics is rotated at a high speed within the drum in order to scan the photoconductive member in the main scanning direction. The drum is moved in its axial direction or subscanning direction in synchronism with the rotation of the rotary optics. This type of scanning mechanism may be applied to a facsimile receiver for a newspaper transmission system, as disclosed in, for example, Japanese Utility Model Laid-Open Publication Nos. 56-68364 and 59-50155. The scanning mechanism may also be applied to an electrophotographic optical printer, as taught in Japanese Patent Laid-Open Publication No. 1-155372 by way of example. However, the problem with this scanning system is that the scanning speed is determined by the rotation speed of the rotary optics which is, in turn, limited by the performance of a motor for driving the optics, and the structural strength of the optics.
In light of the above, use may be made of a multi-beam laser having a plurality of light emitting points. With this kind of laser, it is possible to form a plurality of beam spots on the inner periphery of the drum and modulate them independently of each other. However, when the multi-beam laser is disposed in the rotary optics, it is difficult to supply power thereto. On the other hand, when the laser is disposed in the stationary optics, an image rotating mechanism is necessary in order to prevent scanning lines from intersecting each other on the inner periphery of the drum. While the image rotating mechanism is usually implemented by a Dove prism, the image rotation angle available with such a mechanism is double the rotation angle of the Dove prism, resulting in a complicated rotation mechanism.